<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <link rel="stylesheet" href="css/base.css"/>
    <title></title>
    <style>
        *{margin:0;padding: 0;}
        div{
            width:250px;height:40px;margin-top:15px;
            position:relative;
        }
        input{
            box-sizing: border-box;
            width:250px;
            outline: none;
            padding-left:20px;
            height: 38px;
            font-size: 14px;
            border: 1px solid #eee;
            border-radius: 40px;
            background: #eee;
        }
        button{
            border:none;
            cursor: pointer;
            width:24px;
            height:24px;
            display: block;
            z-index: 100;
            position:absolute;
            top:50%;
            margin-top:-12px;
            right:5px;
            background:url(images/icon4.png) no-repeat
        }
        input:focus div{width:300px;}
    </style>
</head>

<body>
<div class="center">
    <input type="text" placeholder="搜素"/>
    <button></button>
</div>
</body>
<!--结束标签-->
</html>